Simplified plasma channel formation model for the electrical discharge machining process

Electrical discharge machining (EDM) is a controlled metal-removal process used to remove metal by means of electric spark erosion. In this process, a high voltage is applied between two electrodes and the breakdown of the dielectric is initiated by moving the electrode towards the work piece. This movement increases the electric field in the gap until it reaches the breakdown value. When the breakdown occurs, the voltage falls and the current rises abruptly. The presence of a current is possible at this stage because the dielectric has been ionized and a plasma channel (the bubble) has been created between the electrodes. The pre-breakdown phase in the dielectric is characterized by the generation of numerous small bubbles that promote the breakdown process. In this paper a new model for the formation of the plasma channel (bubble) is proposed. This model considers a mixture gas/steam inside the bubble, the shell of the bubble (if any), and the fluid surrounding the bubble. The gas is modeled as ideal and the bubble is initially modeled only as a cavity within the fluid. It is assumed that the bubble is cylindrically symmetric and that the bubble oscillations preserve this symmetry. This assumption is made for the case of a single bubble far from any boundaries, and in a low amplitude-driving field. In these models the center point of the bubble remains stationary at the origin because of the cylindrical symmetry about the origin. The proposed model is evaluated using experimental data and existing models.